Capacitive humidity sensors are mainly used for measuring the air humidity, such as, for example, in air-conditioning technology for controlling an interior environment. Generally, they are thin-film sensors whose measured quantity is the moisture-dependent capacitance of the polymer layer between the electrodes. Sensors of this type are described, for example, in H.-R. Traenkler, E. Obermeier, “Sensortechnik, Handbuch fuer Praxis und Wissenschaft” (“Sensor Technology, Handbook for Practice and Science”), Springer Verlag, 1998, pages 1247 to 1250.
In order to be able to measure the capacitance of the sensor, oscillator circuits are often used in which the capacitive sensor is the frequency-determining member. In this case, a value for the air humidity may be ascertained simply by measuring the oscillator frequency and assigning a humidity value to the result of the measurement. Examples for such circuits in the field of moisture measurement can be found, for example, in H. Lemme, “Sensoren in der Praxis” (“Sensors in Practice”), Franzis-Verlag, 1993, pages 327 to 336.
U.S. Pat. No. 5,844,138 describes a device for measuring humidity, in which a capacitive humidity sensor is the frequency-determining element of an oscillator. The instantaneous air humidity is ascertained by measuring the oscillator frequency and converting it into a humidity value with the aid of calibration data stored in a memory unit.
Particularly in an industrial environment, parasitic parallel resistances, caused by contamination of the sensor connections or by material properties of the sensor itself, appear at the capacitive humidity sensor and also influence the oscillator frequency of the measuring circuit, and thereby falsify the measurement result. Such changes of the impedance connected to the oscillator are not described as being taken into account in U.S. Pat. No. 5,844,138.
The measuring error which comes about due to the parallel connection of a parasitic resistance to the capacitive humidity sensor can be corrected if not only the capacitance, but also the impedance of the measuring sensor is measured, since phase information is also acquired at the same time, and the error produced by the parasitic parallel resistance can be eliminated. However, circuit configurations for measuring impedance are quite complicated and cost-intensive.
European Published Patent Application No. 1 411 349 describes a method for recording the air humidity, in which the charging duration and/or discharging duration of a capacitive humidity sensor is recorded via two different measuring resistors, and the capacitance of the sensor is calculated from the results. In this case, the capacitive sensor is modeled by a parallel connection of an ideal capacitor with an ohmic resistance, both the resistance value and the capacitor value being a function of the moisture and the temperature.